1. Field of the Invention
The invention relates generally to a method and apparatus of thermal imaging used to determine surface and subsurface properties and structure of a sample material. More particularly, the invention deals with a method of dynamic thermal imaging called optical beam deflection wherein an optical probe beam scans the sample surface through a thermal lens generated in the fluid (liquid or gas) adjacent to this surface. An excitation beam heats a localized area on the sample surface generating the thermal refractive lens. The vectorial deflection of the optical probe beam is measured as it scans across the thermal lens. This in turn provides information about properties and features found in the surface and subsurface layers of the sample.
2. Description of the Prior Art
It is well known that the surface temperature of a heated object depends upon its thermal and structural properties (and its optical properties when light is used as a heating source). Several methods of thermal imaging have been used to determine the properties of a sample as a function of the surface temperature profile. Several of the more significant methods include radiometric thermal imaging, photoacoustic imaging and piezoelectric photo imaging.
U.S. Pat. No. 3,504,524, issued to D. R. Maley, uses a radiometric method of thermal imaging wherein the surface temperature is detected by observing localized changes in thermal radiation emitted by the sample. The patent teaches the heating of a sample and the use of a radiometer to detect the thermal energy emitted. The radiometer is aimed at one particular area of the sample surface and locates defects by detecting changes in thermal energy levels. However, problems have been experienced with detector sensitivity, dynamic range and signal discrimination.
Another method of thermal imaging is called photoacoustic imaging. For this method the test sample is placed in an enclosed cell which is filled with a gas. The sample is heated and boundary heating between the sample and the gas produces a pressure wave in the cell which is detected using a sensitive microphone. The method has several serious drawbacks: (1) it requires an enclosed cell which limits the size and type of samples that can be used; (2) it can only detect the average temperature over the entire sample surface, and (3) since the heat source is not localized, three-dimensional heat flow is not significant and less information on surface and subsurface features is available.
Another thermal imaging technique uses a piezoelectric detection method wherein a sample is heated locally, generating stress waves in the specimen which are caused by local thermal expansion. A piezoelectric transducer, bonded to the sample or in contact with the sample via some coupling fluid, detects the stress waves. U.S. Pat. No. 3,222,917, issued to W. Roth, discloses an apparatus wherein a thermal pulse is applied to a localized region and thermal effects are detected by a "pickup transducer" some distance from the thermal pulse source. This method also has several disadvantages: (1) contact is required between the testing device and sample; (2) where large samples are tested, acoustic phase delays are present which must be considered; and, (3) the method does not have the advantages of localized detection unless some type of acoustic lens is used in a manner of the "photoacoustic microscope".
Although the prior art methods recognize the usefulness of thermal imaging to determine structural characteristics of a sample, they were mainly concerned with one-dimensional heat flow analysis. The belief in the prior art, as summarized in an article entitled "Scanned Image Microscopy" by D. Fournier and A. C. Boccara, published by Academic Press (1980), was that three-dimensional diffusion effects were insignificant to thermal imaging.